A mass data storage library is a collection of data storage elements for storing media that record data, data transfer elements for reading the media, and medium transport elements for moving the media between the data storage elements and data transfer elements. The mass storage library is used to store large volumes of data which may be retrieved quickly and efficiently. Examples of data media include cassettes, floppy disks, diskettes, optical disks, compact disks, and tapes which reside in data cartridges; examples of data storage elements include fixed storage shelves or racks and rotatable storage towers; examples of data transfer elements include cartridge processors which are operable to access (read/write) the data media in the cartridges. The medium transport element is typically called an accessor, which includes two primary components: the transport mechanism and the gripper assembly.
In the conventional mass data storage library, a library controller is used to control all library activities. Motion is initiated by a host or operator request to the library controller, which evaluates sensor input and issues motion control signals to the various library hardware elements.